The arrests came after Syria vowed on Saturday to hold "free and transparent" elections by the end of 2011 as Arab states in the Gulf joined a chorus of Western pressure over its deadly suppression of anti-regime protests.

Meanwhile, rallies kicked off after the evening Taraweeh prayers in al-Hajar al-Aswad, al-Kiswah and Maadimiyat al-Sham in support of Hama and Deir al-Zour that are under the Syrian army besiege.

A doctor by training, the 47-year-old is one of the most high-profile members of the opposition in Damascus.

In 2000, Bunni was one of the prime movers of the short-lived "Damascus Spring" amid hopes for reform that followed Bashar Assad's accession to the presidency after the death of his father Hafez.

In May, rights group Amnesty International said veteran dissident Bunni was one of several rights activists in Syria "forced into hiding after receiving threats from Syrian authorities."

Bunni was one of 12 signatories of the 2005 Damascus Declaration, which called for democratic change in Syria, who were sentenced to 30 months in jail in October 2008 for "damaging the state." He was last released in June 2010.

The Syrian government has sought to crush the democracy movement with brutal force, killing around 1,650 civilians and arresting thousands of dissenters, according to the Syrian Observatory for Human Rights.

Abdel Rahman of the Syrian Observatory said around 250 tanks and armored cars were deployed in four districts of Deir al-Zour on Saturday.

The tanks were also posted around the airport in Deir al-Zour, many of whose residents started to flee the city from Wednesday, fearing imminent military action.

In Homs, "many armored cars and other army vehicles have been posted in the Bab al-Sibaa district," Abdel Rahman said, adding that activists in the city reported gunfire from early morning.